| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 208 |
| 4 | 1540 |
| 5 | 11344 |
| 6 | 83188 |
| 7 | 607408 |
| 8 | 4416580 |
| 9 | 31986064 |
| 10 | 230784148 |
| 11 | 1659338608 |
| 12 | 11892395620 |
| 13 | 84983496784 |
| 14 | 605698755508 |
| 15 | 4306834677808 |
| 16 | 30560156566660 |
[1,4,28,208,1540,11344,83188,607408,4416580, 31986064,230784148,1659338608,11892395620, 84983496784,605698755508,4306834677808, 30560156566660] |